Rigid contact lenses are an important segment of the contact lens market because they alone can be manufactured with the precision required to correct the more severe vision defects, like astigmatism.
Originally, hard lenses were made from poly(methyl methacrylate) and had no oxygen permeability; Rigid-Gaspermeable Contact Lenses, RGP's, were first developed in the early 1970's to provide the possibility of extended wear and greater comfort and are described in U.S. Pat. No. 3,808,178; they are copolymers of oligosiloxanyl-alkyl methacrylates with oxygen permeabilities of 10 to 25 barrers; subsequently RGP polymers were described, for example in U.S. Pat. Nos. 4,486,577; 4,605,712; 4,734,475 and 4,923,906, which are based not on oligosiloxane-mono-methacrylates, but on polymeric dimethylsiloxane-di- and poly-methacrylates and which are characterized by greater toughness and oxygen permeabilities DK of up to 35 barrers.
While these levels of DK are high enough to ensure an oxygen flux to the cornea, which will reduce the incidence and severity of oxygen deficiency, for truly extended-wear use and over-night use of an RGP, permeabilities much higher are required in order to reduce corneal swelling to a minimum. This has more recently been accomplished by including fluorinated comonomers into the polymer, in addition to the silicone containing components; such compositions have been described in JP 82-130836, in which trifluoroethyl methacrylate is added to an oligosiloxanyl-alkyl methacrylate type RGP formulation, and in U.S. Pat. No. 4,486,577, where hexafluoroisopropyl methacrylate is combined with the poly(dimethylsiloxane)-macromer type RGP formulation.
With such combination polymers O.sub.2 DK values of up to 60 have been achieved. Attempts however to further increase the permeability by increasing the proportion of silicone and fluorine containing monomers conflict with the need to keep other physical properties, like hardness and dimensional stability and toughness at a useful level. If, for example, in using the oligosiloxanyl-monomer approach the amount of fluorinated monomers is increased over 30%, an increasingly brittle polymer is obtained, which despite its high O.sub.2 -permeability cannot be machined or handled as a contact lens without breaking it. On the other hand, if in using the polysiloxy macromer approach either the fluorinated monomer or the siloxane-macromer is increased above a certain percentage, phase separation can led to unacceptable haziness and the polymers hardness is reduced to a level where machining and polishing the polymer and lens become impossible and dimensional stability is sacrificed.
It has now unexpectedly been discovered, that if in a fluorine and silicone containing RGP formulation 1-15% of a poly(dimethylsiloxane) macromer is incorporated, the resulting polymers possess a combination of toughness, hardness and oxygen permeability which is far superior to what can be attained otherwise.
Similiar polymers are described in U.S. Patent Nos. 4,486,577; 4,605,712; 4,734,405 and 4,923,906, but in all cases do these polymers contain at least 15% by weight of the poly(siloxane)-macromer, typically more than 25%, and the polymers have therefore a poorer combination of properties, than the polymers of the instant invention. No hard contact lens materials have been described which combine units of an oligosiloxane methacrylate, a fluorinated methacrylate and a poly(siloxane)-macromer, and which possess as high an oxygen permeability and hardness as the polymers of the instant invention.